Name: 2d nmr spectra and signal mapping
Uploaded: 2024-12-05T18:32:26.000Z
Duration: 1 min 12 s
Channel: JoVE

In 2-dimensional or 2D NMR, introducing a second frequency axis enables the identification of coupled spins, yielding connectivity information in a molecule.
First, one of the coupled nuclei is excited during the preparation period.
This is followed by a systematically incremented evolution or wait period, t1, during which no data is observed.
During the mixing period, a second pulse transfers the magnetization from the excited spins to the coupled nuclei.
Finally, the magnetization of the coupled spins is detected during the acquisition time t2 for each t1.
Time domains, t1 and t2, are Fourier transformed to yield the frequency domains F1 and F2, where F1 is usually a chemical shift scale.
Coupling between the same kind of nucleus is studied by homonuclear 2D experiments, whereas heteronuclear 2D experiments investigate the correlation between different nuclei.
2D spectra are generally represented as contour plots where the size of the circles is proportional to the signal intensity.

two-dimensional (2d) nmr: overview

Two-Dimensional (2D) NMR: Overview

The 1D NMR spectrum of large and complex molecules like natural products has complicated splitting patterns and overlapping signals, which can be easily interpreted using 2-dimensional (2D) NMR. Unlike 1D NMR, 2D NMR has two frequency axes that provide the coupling information between the nucleus A and nucleus B in a molecule. The process from which 2D spectra are obtained has four steps.
The first step is the preparation period, during which nucleus A is excited with a radiofrequency pulse. The second step is the evolution or wait period t1, which is incremented systematically, and no data is observed during this period. The next step is the mixing period, in which a second radiofrequency pulse is introduced, which transfers the magnetization to the spins of nucleus B. In the last step, the magnetization of nuclear spins from B is detected during the acquisition time t2 for each t1. Time domains t1 and t2 are Fourier transformed into frequency domains f1 and f2, and these correspond to two dimensions of 2D spectra, while the third dimension is the intensity. In 2D experiments, coupling between the same kind of nucleus is called homonuclear 2D experiments, whereas coupling between two types of nucleus is called heteronuclear 2D experiments. 2D spectra can be displayed as stacked or contour plots. When presented as stacked plots, 2D spectra are difficult to interpret. The contour plot is the horizontal cross-section of a stacked plot where the size of the circles shows the signal intensity. Its interpretation is straightforward and it is preferred for data analysis.

The 1D NMR spectrum of large and complex molecules like natural products has complicated splitting patterns and overlapping signals, which can be easily ...

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Analytical Chemistry

Unit 1

Advanced Nuclear Magnetic Resonance Spectroscopy

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KEY TERMS AND CONCEPTS

2d nmr spectra and signal mapping

Two-dimensional (2D) NMR helps make sense of crowded NMR data from large and complex molecules, such as natural products. A 1D NMR spectrum can have overlapping signals and complicated splitting patterns. 2D NMR adds a second frequency axis, which helps show the coupling information between nucleus A and nucleus B in a molecule.

2D spectra are built in four steps. First is the preparation period, when nucleus A is excited with a radiofrequency pulse. Next is the evolution, or wait, period t1. During t1, no data is observed, and the period is increased in a systematic way. The third step is the mixing period, when a second radiofrequency pulse transfers magnetization to the spins of nucleus B. The last step is the acquisition time t2, when magnetization from nuclear spins of B is detected for each t1.

The time domains t1 and t2 are then Fourier transformed into the frequency domains f1 and f2. These frequency axes form two dimensions of the 2D spectrum. The third dimension is intensity. In 2D experiments, coupling between the same kind of nucleus is called a homonuclear 2D experiment. Coupling between two different kinds of nucleus is called a heteronuclear 2D experiment.

2D spectra can be displayed as stacked plots or contour plots. Stacked plots are harder to interpret because the data are layered. A contour plot is a horizontal cross-section of a stacked plot. In a contour plot, the circle size shows signal intensity. This format is easier to read and is preferred for data analysis.

2D NMR Spectra and Signal Mapping

&sup1;h nmr of conformationally flexible molecules: temporal resolution

h-nmr-of-conformationally-flexible-molecules-temporal-resolution

¹H NMR Ring Flips and Signal Averaging

¹H NMR can show how fast molecular motion changes the signals we see. In cyclohexane, the chair form flips rapidly between two equivalent chair conformers at room temperature. This happens about 10^5 times per second, and the two shapes stay in equilibrium. During ring flipping, an axial proton becomes equatorial, and an equatorial proton becomes axial. These changes are too fast to detect on the NMR timescale. As a result, the NMR spectrometer cannot tell the axial and equatorial protons...

&sup1;h nmr of conformationally flexible molecules: variable-temperature nmr

h-nmr-of-conformationally-flexible-molecules-variable-temperature-nmr

Cyclohexane Ring Flipping in Variable-Temperature NMR

Cyclohexane ring flipping can be tracked with variable-temperature NMR. The experiment helps distinguish the axial and equatorial protons in a deuterium-substituted cyclohexane sample. Deuterium is used to replace eleven of the twelve protons. This reduces possible peak splitting from spin-spin coupling between neighboring protons. The remaining proton shifts between the axial and equatorial positions as the chair form of cyclohexane changes. At room temperature, ring flipping is very fast, so...

&sup1;h nmr of labile protons: temporal resolution

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OH Proton Exchange in Ethanol NMR

The OH proton in alcohols can shift and split differently in a proton NMR spectrum. Its position usually falls around 2 to 5 ppm, but the exact value depends on the alcohol and the sample conditions. This proton is labile, which means it can exchange rapidly with other molecules. Hydrogen bonding to oxygen or nitrogen changes the electron density around the proton. That change affects the chemical shift seen in the proton NMR spectrum. The exchange rate also affects spin-spin coupling between...

&sup1;h nmr of labile protons:  deuterium (&sup2;h) substitution

h-nmr-of-labile-protons-deuterium-h-substitution

Using D2O to Identify OH Protons

Deuterium substitution can help identify labile protons in ¹H NMR spectra. Labile protons are hydrogens that can be replaced by another hydrogen isotope. In this lesson, deuterium is used to simplify the spectrum of ethanol. Deuterium, or ²H, is one of the three isotopes of hydrogen. Its nucleus contains one proton and one neutron. When D₂O is added to a pure dry ethanol solution, the labile proton on the –OH group is replaced by deuterium. A proton NMR spectrum does not show signals from...

nuclear overhauser enhancement (noe)

Using NOE to Map Nearby Nuclei

Nuclear Overhauser enhancement, or NOE, shows how irradiation of one spin-active nucleus can change the signal of nearby nuclei. These nuclei do not have to be chemically bonded or connected through J-coupling. The effect comes from through-space interactions between nuclear spins. NOE gets weaker as the distance between nuclei increases. It is generally not observed beyond about 4 angstroms. Dipole-dipole interactions between neighboring spin-active nuclei transfer nuclear spin polarization,...

insensitive nuclei enhanced by polarization transfer (inept)

insensitive-nuclei-enhanced-by-polarization-transfer-inept

NMR Signal Boosting with INEPT Pulses

INEPT is an NMR method that boosts weak signals from low-abundance nuclei. It is used to detect nuclei such as carbon-13 and nitrogen-15 in small molecules. The goal is to make these hard-to-see nuclei easier to observe in an NMR spectrum. The technique works by transferring polarization from a more abundant nucleus, usually hydrogen-1, to the nucleus of interest. Hydrogen-1 has higher receptivity, so it can provide the starting signal boost. Spin coupling then helps move that polarization to...

double resonance techniques: overview

Spin Decoupling in NMR Spectroscopy

Spin decoupling is a key use of double resonance in nuclear magnetic resonance (NMR) spectroscopy. It uses two radiofrequency pulses or frequencies at the same time to act on two different nuclear spins. This makes the spectrum simpler and easier to read. In spin decoupling, one type of nucleus is selectively suppressed while the signal from another nucleus is observed. The radiofrequency pulse sequence removes the coupling with one nuclide and can greatly improve resolution. As a result, the...

Two-dimensional (2D) NMR helps make sense of crowded NMR data from large and complex molecules, such as natural products. A 1D NMR spectrum can have overlapping signals and complicated splitting patterns. 2D NMR adds a second frequency axis, which helps show the coupling information between nucleus A and nucleus B in a molecule. 2D spectra are built in four steps. First is the preparation period, when nucleus A is excited with a radiofrequency pulse. Next is the evolution, or wait, period t1.

2d nmr: overview of homonuclear correlation techniques

2d-nmr-overview-of-homonuclear-correlation-techniques

2D NMR Correlation Paths in Molecules

2D NMR homonuclear correlation techniques show how nuclei of the same type connect within a molecule. In nuclear magnetic resonance (NMR) spectroscopy, these methods reveal scalar couplings, which are interactions between nearby nuclei through bonds. That information helps identify which atoms are linked and how a molecule is put together. COSY is the main homonuclear correlation experiment. COSY90 is the standard 2D COSY method and uses a second 90-degree pulse to improve sensitivity and...

2d nmr: homonuclear correlation spectroscopy (cosy)

2d-nmr-homonuclear-correlation-spectroscopy-cosy

Reading Proton Coupling with COSY Maps

COSY is a 2-dimensional NMR method that shows which protons are coupled to each other. It is used to spot geminal and vicinal coupling in a sample. This makes it helpful for reading proton relationships in complex spectra. The ethyl acetate example shows how COSY is built from the 1D proton NMR spectrum. The same chemical shift scale appears on both the vertical and horizontal axes. The peaks along the diagonal are called diagonal peaks, and they match the three peaks seen in the 1D proton...

2d nmr: overview of heteronuclear correlation techniques

2d-nmr-overview-of-heteronuclear-correlation-techniques

2D NMR Correlation Peaks and Bond Links

Heteronuclear correlation spectroscopy is a 2D NMR method that links different nuclei, usually a proton and an X-nucleus such as carbon-13 or nitrogen-15. It helps scientists study the structure and composition of complex chemical compounds. In a heteronuclear correlation spectrum, one axis shows X-nucleus chemical shifts and the other axis shows the proton spectrum. Cross-peaks mark which protons are connected to which X-nuclei. The main heteronuclear correlation experiments are HETCOR, HSQC,...

2d nmr: heteronuclear single-quantum correlation spectroscopy (hsqc)

2d-nmr-heteronuclear-single-quantum-correlation-spectroscopy-hsqc

HSQC NMR Maps Carbon-Proton Connections

HSQC NMR maps the direct links between hydrogen atoms and the carbons they are attached to. It is a 2D NMR method, so it shows the data on two axes instead of one. The experiment is similar to heteronuclear correlation spectroscopy, or HETCOR, but HSQC is more sensitive. In an HSQC spectrum, the proton chemical shift appears on the horizontal F2 axis. The 13C chemical shift appears on the vertical F1 axis. The spectrum also includes the matching proton and 13C spectra. The contour plot does...

key terms and concepts

Learn how 2D NMR turns crowded spectra into clear signal maps using t1 and t2 periods, frequency axes, and contour plots.

2D NMR Spectra and Signal Mapping | JoVE K12

analytical chemistry

JoVE Core Analytical Chemistry simplifies complex chemical principles by breaking them down into clear, engaging animations, making topics from SI Units to NMR spectroscopy more accessible and straightforward. Additionally, the scientist-in-action videos highlight the application of these concepts in real-world research conducted in laboratories globally.

2D NMR Spectra and Signal Mapping

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